Hitting a 10 ppb oxygen target for injection water is non‑negotiable in enhanced oil recovery. Operators are choosing between tall, power‑hungry vacuum deaeration towers and compact, chemical scavenger skids—often ending up with both.
In upstream water injection, every part per billion (ppb, a concentration unit) of dissolved oxygen (O₂) matters. Typical offshore targets are about 10 ppb before injection to the reservoir (ScienceDirect). A two‑stage vacuum deaeration system on one offshore project reached roughly 50 ppb—impressive, but still above the spec (docslib.org).
The result is a pragmatic industry default: let mechanical systems remove the bulk of the oxygen load, then “polish” with chemical oxygen scavengers to close the gap (ScienceDirect). Each approach has distinct cost, footprint, and reliability trade‑offs offshore.
Vacuum deaeration: mechanism and performance
Vacuum deaerators strip dissolved oxygen by lowering water pressure—Henry’s law (gas solubility falls as pressure drops)—usually in packed towers served by vacuum pumps or ejectors. Multi‑stage towers can drive O₂ into the sub‑ppm range (tens of ppb) (docslib.org).
In practice, the last mile is tough. A two‑stage offshore system achieved about 50 ppb O₂ (docslib.org), while the target sits near ~10 ppb (ScienceDirect). That’s why many designs pair a vacuum tower with a small chemical “polish” downstream.
Gas removal scope and operating implications
Vacuum systems avoid large chemical inventories and directly vent oxygen and other dissolved gases—CO₂ and N₂ included—in one unit. Performance can be boosted by combining vacuum with a stripping gas (an inert lift gas), when available. The continuous operation suits large flow rates without the mixing complexity of reagents.
The trade‑offs are physical and electrical. Offshore designs flag “high system footprint/volume and weight” as constraints that may limit vacuum towers on space‑constrained platforms (docslib.org). Liquid‑ring vacuum pumps and associated equipment draw significant power—“high power costs”—and require tight seals and vigilant maintenance to prevent air ingression (PetroWiki).
Capital intensity and deck impact
Capital cost is substantial: multi‑stage columns, vacuum pumps, and controls frequently run into multi‑million‑dollar packages for large flows. A medium‑size seawater deaerator can stand many meters tall and weigh tens of tons, demanding scarce deck area and weight allowance (docslib.org).
Over the life of field, the operating expense (OPEX) is a balance of electricity—often in the hundreds of kW—and maintenance. That spend competes with the variable cost of chemicals on an all‑chemical design (PetroWiki).
Chemical oxygen scavengers: dosing and side effects
Chemical scavengers react stoichiometrically (one reagent reacts per oxygen molecule) to convert O₂ into innocuous products. Common oilfield choices include sodium sulfite (Na₂SO₃), sodium bisulfite (NaHSO₃), and ammonium bisulfite (NH₄HSO₃) (Scribd). Operators typically inject a concentrated solution in the deaerator sump or downstream of gas stripping. Properly dosed, scavengers can push residual O₂ to ppb‑level—or better—without gas‑phase stripping (pdfcoffee.com).
Compactness is the appeal: a chemical package of metering pumps and storage tanks occupies only a few square meters compared to a full tower. In many builds, simple metering is handled by a dedicated dosing pump, and storage plus controls are bundled as supporting equipment. Product selection commonly falls under oilfield reagents such as oxygen and H₂S scavengers, within broader oilfield chemical programs.
The math scales with flow and inlet oxygen. A typical rule of thumb is 1–2 g of 45% bisulfite solution per 10 L of water per mg/L of O₂ (ChE Resources). For example, treating 10 m³/h of water at 8 mg/L dissolved O₂ takes on the order of 20–40 kg of 45% sulfite per hour. With sodium bisulfite prices around $400–$600 per tonne (chemanalyst.com), daily dosing on large platforms runs into thousands of dollars.
The chemistry brings caveats. These solutions are corrosive/acidic and need compatible storage (Scribd). Ammonium bisulfite requires no catalyst (Scribd) but can stimulate sulfate‑reducing bacteria, risking souring (H₂S) and microbiologically influenced corrosion (MIC) (Scribd). Still, dosing systems are simple and rely on well‑understood pumps and controls, with standard redundancy (spare pumps, valve bypasses) common offshore.
Cost, footprint, and reliability comparison
Offshore treatment trains frequently deploy both approaches: a vacuum or gas‑stripping stage to remove the bulk of oxygen, then chemicals for polishing to <10 ppb (ScienceDirect; SlideShare). On capital (CAPEX), vacuum towers dominate. One case study found that replacing a two‑stage vacuum tower with membranes—and eliminating the scavenger—saved substantial capital (docslib.org).
Operating expense (OPEX) often flips the balance. Vacuum units carry significant electrical demand (hundreds of kW) and maintenance exposure; chemical purchases, while sizeable, can remain below the cost of running large vacuum plants in certain duty ranges (PetroWiki). The exact crossover depends on flow and inlet O₂.
Footprint and weight are decisive offshore. A vacuum deaerator for seawater at hundreds of m³/h can be many meters tall and tens of tons, consuming prime deck real estate (docslib.org). A chemical skid—pumps, tanks, controls—can fit within a few square meters, a clear advantage when platform space and crane capacity are tight.
Reliability is a study in trade‑offs. Vacuum towers have few moving parts beyond pumps, but any seal failures or pump downtime can allow air ingress and degrade performance (PetroWiki). Chemical systems depend on continuous dosing; an empty tank or tripped pump causes O₂ rebound. Standard mitigations include redundant dosing skids and practices like nitrogen blanketing on tanks to minimize O₂ pickup.
Practical configuration for offshore EOR
In enhanced oil recovery (EOR), where injection water quality underpins reservoir integrity, the pattern is consistent: vacuum towers handle the heavy lifting without chemical reagents, then scavengers clean up residual O₂ to the <10 ppb spec (ScienceDirect; pdfcoffee.com). Choices bend to project specifics: very large flows favor vacuum to minimize chemical volumes, while tight decks and weight limits favor compact chemical packages.
The bottom line from the field data holds across cases: vacuum towers deliver very low O₂ but at high CAPEX, energy, and maintenance cost (PetroWiki; docslib.org). Chemical scavengers achieve ultra‑low oxygen with minimal space but impose ongoing reagent cost and biological/corrosion risks (Scribd). That is why “vacuum plus polish” remains the offshore workhorse (SlideShare).
Sources embedded above: petrowiki.spe.org; pdfcoffee.com; www.sciencedirect.com; fr.scribd.com; fr.scribd.com; fr.scribd.com; docslib.org; docslib.org; docslib.org; www.cheresources.com; www.chemanalyst.com; www.slideshare.net.
